The present invention relates to a motor-driven power steering apparatus using a motor-driven actuator for generating an auxiliary steering force, and more particularly to a motor-driven power steering apparatus which is suitable for automobiles.
As a power steering apparatus for automobiles, a conventional oil pressure type steering apparatus has been primarily used. Recently, a motor-driven power steering apparatus was proposed, since it controls various items more easily and it is a simpler piece of equipment within an automobile. For example, a motor-driven power steering apparatus is disclosed in FIGS. 2 and 4 of Japanese Patent Application Laid-Open No. 59-70257 laid open on Apr. 20, 1984.
Common construction of a motor-driven power steering apparatus is shown, for instance, by FIG. 18.
Referring to FIG. 18, 1 denotes a steering wheel. 2 denotes a torque sensor which is applied to a rotational axis of the handle 1. 3 denotes a steering gear (steering mechanism). 4 denotes a driven vehicle wheel. 5 denotes a DC motor for generating an auxiliary steering force. 6 denotes a clutch to cut the control force of the steering wheel 1. 7 denotes a reduction gear. 8 denotes a controller (control circuit). 9 and 10 denote switching transistors. 11 denotes a device for detecting current through the DC motor 5. 12 denotes a battery. 13 denotes an alternator. 14 and 15 are flywheel diodes. 50 denotes an armature. 51 and 52 are magnetic coils.
In operation of the steering wheel, an operating torque applied to the steering wheel 1 is detected by the torque sensor 2, and fed to the controller 8. The controller 8 supplies pulse output signals, which have different duty ratios corresponding to detected signals of the torque sensor 2, to the transistor 9 or 10 for controlling the motor 5 by chopper control. As a result the motor 5 transmits required torque to the driven or turning wheel 4 through the clutch 6 and the reduction gear 7 assists the steering force of the steering wheel 1.
The motor 5 is a series-wound D.C. motor having a field winding 51, for clockwise rotation an a field winding 52 for anticlockwise (or counterclockwise) rotation. A power source for driving the motor is a battery 12 mounted on an automobile and charged by the alternator 13.
With the foregoing construction, a steering angle of the driven or turning wheel 4 is imparted by a composite steering force of the steering gear 3 from the steering wheel 1 and an auxiliary steering force transmitted through the reduction gear 7 from the motor 5. Accordingly, the motor 5 generates the auxiliary steering torque corresponding to an operating torque T, and a power steering function can be obtained. At this time, the controller 8 introduces a signal I.sub.M representing a current I which flows to the motor 5 through the current detector 11, and a feedback control can be performed in such a manner that a predetermined current I corresponding to the torque T is correctly supplied to the motor 5.
However, we have found that such a conventional power steering apparatus has the disadvantage of not providing back-up when an abnormality is generated in the torque sensor, but of merely stopping the power steering function by cutting the clutch 6 from the motor 5.
As explained above, when an abnormality occurs in a torque sensor, the conventional power steering apparatus immediately stops functioning. Concerning this point, the conventional power steering apparatus has serious deficiencies in reliability.